Gallium nitride (GaN) homoepitaxy is not achievable to date due to the absence of native GaN substrates. Therefore, it is necessary to resort to heteroepitaxy, that is, growth on a substrate of a different nature, to grow GaN layers.
For this purpose, substrates such as silicon or silicon carbide are particularly appreciable due to their slight difference in lattice parameter with GaN.
However, such substrates present the disadvantage of presenting a coefficient of thermal expansion (CTE) that is substantially less than that of gallium nitride, such that, during cooling after the epitaxy, tensile stresses are generated in the gallium nitride layer.
The thicker the gallium nitride layer, the larger the stresses. When the stresses exceed a certain threshold, the material tends to relax by forming cracks.
Cracks are macroscopic damages in the layer, i.e., discontinuities that appear at the surface of the layer, making it unusable for manufacturing electronic devices.
A method for manufacturing a crack-free gallium nitride monocrystalline layer, on a substrate likely to generate tensile stresses in the layer is known from WO 01/95380.
With reference to FIG. 1, this method consists of successively forming on a substrate 10:                a nucleation layer 20 or buffer layer        a first layer 30 of GaN        a monocrystalline intermediate layer 40 in a material whose lattice parameter is smaller than that of gallium nitride, and presenting a thickness of between 100 and 300 nm        the thick layer 50 of GaN.        
The function of the intermediate layer 40 is to be a seed layer for the growth of gallium nitride. Gallium nitride in fact conforms to the lattice parameter of the material of the layer on which it is formed.
Because of its lower lattice parameter, the intermediate layer 40 imposes, at the deposition temperature, a compressive stress in the overlying gallium nitride layer 50.
This compressive stress offsets the tensile stress generated in the GaN during cooling, due to the difference in thermal expansion coefficient between GaN and the substrate.
According to the prior art, the intermediate layer 40 must present good crystalline quality to enable the epitaxy of a good quality and stressed GaN layer. So that the GaN does not relax at the interface, it is important in fact that the interface between the intermediate layer and the layer of GaN is completely flat.
This method thus enables layers whose thickness may reach approximately 3 μm to date to be manufactured on a silicon substrate.
However, the thickness by which the GaN may grow remains limited since during growth, it partially relaxes by forming dislocations. Beyond a certain thickness, the layer of GaN is thus again in tension and is likely to crack during cooling.
One embodiment of the invention thus enables the formation of gallium nitride layers that are even thicker than those achievable in the prior art. Typically, one seeks to obtain layers with a thickness greater than 2 μm, that may reach 7 μm or more without cracking and preferably with a level of dislocation of less than 5·109 cm−2.
In addition, the more the layer thickness increases, the more a concave curvature in the upper surface of the structure is observed. The tension stress in the GaN layer in fact induces a concave deformation in the silicon substrate. This phenomenon is all the more appreciable the larger the wafer diameter. Now, this lack of flatness is a problem that may turn out to be insurmountable for subsequent technological processes when making electronic or optoelectronic components.
Thus, another embodiment of the invention is to improve the flatness of the thick layer of gallium nitride formed.